1. Field of the Invention
This invention relates generally to formation fluid sampling, and more specifically to an improved formation fluid sampling module, the purpose of which is to bring high quality formation fluid samples to the surface for analysis, in part, by eliminating the xe2x80x9cdead volumexe2x80x9d which exists between a sample chamber and the valves which seal the sample chamber in the sampling module.
2. Description of the Related Art
The desirability of taking downhole formation fluid samples for chemical and physical analysis has long been recognized by oil companies, and such sampling has been performed by the assignee of the present invention, Schlumberger, for many years. Samples of formation fluid, also known as reservoir fluid, are typically collected as early as possible in the life of a reservoir for analysis at the surface and, more particularly, in specialized laboratories. The information that such analysis provides is vital in the planning and development of hydrocarbon reservoirs, as well as in the assessment of a reservoir""s capacity and performance.
The process of wellbore sampling involves the lowering of a sampling tool, such as the MDT(trademark) formation testing tool, owned and provided by Schlumberger, into the wellbore to collect a sample or multiple samples of formation fluid by engagement between a probe member of the sampling tool and the wall of the wellbore. The sampling tool creates a pressure differential across such engagement to induce formation fluid flow into one or more sample chambers within the sampling tool. This and similar processes are described in U.S. Pat. Nos. 4,860,581; 4,936,139 (both assigned to Schlumberger); U.S. Pat. Nos. 5,303,775; 5,377,755 (both assigned to Western Atlas); and U.S. Pat. No. 5,934,374 (assigned to Halliburton).
The desirability of housing at least one, and often a plurality, of such sample chambers, with associated valving and flow line connections, within xe2x80x9csample modulesxe2x80x9d is also known, and has been utilized to particular advantage in Schlumberger""s MDT tool. Schlumberger currently has several types of such sample modules and sample chambers, each of which provide certain advantages for certain conditions.
xe2x80x9cDead volumexe2x80x9d is a phrase used to indicate the volume that exits between the seal valve at the inlet to a sample cavity of a sample chamber and the sample cavity itself. In operation, this volume, along with the rest of the flow system in a sample chamber or chambers, is typically filled with a fluid, gas, or a vacuum (typically air below atmospheric pressure), although a vacuum is undesirable in many instances because it allows a large pressure drop when the seal valve is opened. Thus, many high quality samples are now taken using xe2x80x9clow shockxe2x80x9d techniques wherein the dead volume is almost always filled with a fluid, usually water. In any case, whatever is used to fill this dead volume is swept into and captured in the formation fluid sample when the sample is collected, thereby contaminating the sample.
The problem is illustrated in FIG. 1, which shows sample chamber 10 connected to flow line 9 via secondary line 11. Fluid flow from flow line 9 into secondary line 11 is controlled by manual shut-off valve 17 and surface-controllable seal valve 15. Manual shut-off valve 17 is typically opened at the surface prior to lowering the tool containing sample chamber 10 into a borehole (not shown in FIG. 1), and then shut at the surface to positively seal a collected fluid sample after the tool containing sample chamber 10 is withdrawn from the borehole. Thus, the admission of formation fluid from flow line 9 into sample chamber 10 is essentially controlled by opening and closing seal valve 16 via an electronic command delivered from the surface through an armored cable known as a xe2x80x9cwireline,xe2x80x9d as is well known in the art. The problem with such sample fluid collection is that dead volume fluid DV is collected in sample chamber 10 along with the formation fluid delivered through flow line 9, thereby contaminating the fluid sample. To date, there arc no known sample chambers or modules that address this problem of contamination resulting from dead volume collection in a fluid sample.
To address this shortcoming, it is a principal object of the present invention to provide an apparatus and method for bringing a high quality formation fluid sample to the surface for analysis.
It is a further object of the present invention to provide a method and apparatus of flushing the dead volume fluid from a sample module prior to the collection of a fluid sample in a sample chamber within the sample module.
It is a further object of the present invention to utilize a controllable inlet and outlet fluidly connected to a sample cavity of a sample module to achieve dead volume flushing.
The objects described above, as well as various other objects and advantages, are achieved by a sample module for use in a tool adapted for insertion into a subsurface wellbore for obtaining fluid samples therefrom. The sample module includes a sample chamber for receiving and storing pressurized fluid, and a piston slidably disposed in the chamber to define a sample cavity and a buffer cavity, the cavities having variable volumes determined by movement of the piston. A first flowline is provided for communicating fluid obtained from a subsurface formation through the sample module. A second flowline connects the first flowline to the sample cavity, and a third flowline connects the sample cavity to either the first flowline or an outlet port. A first valve is disposed in the second flowline for controlling the flow of fluid from the first flowline to the sample cavity, and a second valve is disposed in the third flowline for controlling the flow of fluid out of the sample cavity, whereby any fluid preloaded in the sample cavity may be flushed therefrom using the formation fluid in the first flowline and the first and second valves.
In a particular embodiment of the present invention, the sample module further includes a third valve disposed in the first flowline for controlling the flow of fluid into the second flowline. The second flowline of this embodiment is connected to the first flowline upstream of the third valve. The third flowline is connected to the sample cavity and to the first flowline, the latter connection being downstream of the third valve.
The present invention may be further equipped, in certain embodiments, with a fourth flowline connected to the buffer cavity of the sample chamber for communicating buffer fluid into and out of the buffer cavity. The fourth flowline is also connected to the first flowline, whereby the collection of a fluid sample in the sample cavity will expel buffer fluid from the buffer cavity into the first flowline via the fourth flowline. In some embodiments of the present invention, a fifth flowline is connected to the fourth flowline and to the first flowline, the latter connection being upstream of the connection between the first and second flowlines, the fifth flowline permitting manipulation of the buffer fluid to create a pressure differential across the piston for selectively drawing a fluid sample into the sample cavity. The fourth and fifth flowlines thus connect the buffer cavity to the first flowline both upstream and downstream of the third valve. When the present invention is so equipped with the fourth and fifth flowlines, manual valves are preferably positioned in these flowlines to select, uphole, whether the buffer fluid is communicated to the first flowline upstream of the third valve or downstream of the third valve.
The present invention may be further defined in terms of an apparatus for obtaining fluid from a subsurface formation penetrated by a wellbore, comprising a probe assembly for establishing fluid communication between the apparatus and the formation when the apparatus is positioned in the wellbore, and a pump assembly for drawing fluid from the formation into the apparatus via the probe assembly. A sample module is provided for collecting a sample of the formation fluid drawn from the formation by the pumping assembly. The sample module includes a chamber for receiving and storing fluid, and a piston slidably disposed in the chamber to define a sample cavity and a buffer/pressurization cavity, the cavities having variable volumes determined by movement of the piston. A first flowline is placed in fluid communication with the pump assembly for communicating fluid obtained from the formation through the sample module. A second flowline connects the first flowline to the sample cavity, and a third flowline connects the sample cavity to one of the first flowline and an outlet port. A first valve is disposed in the second flowline for controlling the flow of fluid from the first flowline to the sample cavity; and a second valve is disposed in the third flowline for controlling the flow of fluid out of the sample cavity. In this manner, any fluid preloaded in the sample cavity may be flushed therefrom using formation fluid and the first and second valves.
A particular embodiment of this inventive apparatus further includes a pressurization system for charging the buffer/pressurization cavity to control the pressure of the collected sample fluid in the sample cavity via the floating piston. The pressurization system preferably includes a valve positioned in a pressurization flowline connected for fluid communication with the buffer/pressurization cavity of the sample chamber. The valve is movable between positions closing the buffer/pressurization cavity and opening the buffer/pressurization cavity to a source of fluid at a greater pressure than the pressure of the formation fluid delivered to the sample cavity.
In one application of this embodiment, the pressurization system controls the pressure of the collected sample fluid within the sample cavity during collection of the sample from the formation, and it utilizes wellbore fluid for this purpose.
In another application of this embodiment, the pressurization system controls the pressure of the collected sample fluid within the collection cavity during retrieval of the apparatus from the wellbore to the surface, and it utilizes a source of inert gas carried by the apparatus for this purpose.
It is preferred that the inventive apparatus is a wireline-conveyed formation testing tool, although the advantages of the present invention are also applicable to a logging-while-drilling (LWD) tool such as a formation tested carried in a drillstring.
The present invention further provides a method for obtaining fluid from a subsurface formation penetrated by a wellbore, comprising the steps of positioning a formation testing apparatus within the wellbore, and establishing fluid communication between the apparatus and the formation. Once fluid communication is established, fluid from the formation is induced to move into the apparatus. A sample of the formation fluid is then delivered to a sample cavity of a sample chamber carried by the apparatus, and at least a portion of the delivered formation fluid is moved through the sample cavity to flush out at least a portion, and preferably all, of a fluid (typically water) precharging the sample cavity. After this flushing step, a sample of the formation fluid is collected within the sample cavity. At some point following the collection of a formation fluid sample, the apparatus is withdrawn from the wellbore to recover the collected sample or, in the case of a multi-sample module, plurality of samples.
In a particular embodiment of the inventive method, the flushing step is accomplished with flow lines leading into and out of the sample cavity, and each of the flow lines is equipped with a seal valve for controlling fluid flow therethrough from a command at the surface. The fluid precharging the sample cavity, as well as the flow lines between the sample cavity and the seal valves controlling access thereto, may be flushed directly out to the borehole or may be flushed into a primary flow line within the apparatus for subsequent use in another module or later discharge to the borehole.
Preferably, the inventive method further includes the step of maintaining the sample collected in the sample cavity in a single phase condition as the apparatus is withdrawn from the wellbore.
It is also preferred in the inventive method that the sample chamber include a floating piston slidably positioned therein so as to define the sample cavity and a buffer/pressurization cavity. Among other things, this permits the buffer/pressurization cavity to be charged to control the pressure of the sample in the sample cavity.
The buffer/pressurization cavity is charged, in one application, with a buffer fluid. The buffer fluid is expelled from the buffer/pressurization cavity in this application by movement of the piston as the formation fluid is delivered to and collected within the sample cavity. In the preferred embodiment of this inventive method, the expelled buffer fluid is delivered to a primary flow line within the apparatus for subsequent use in another module or later discharge to the borehole.
Fluid movement from the formation into the apparatus is induced by a probe assembly engaging the wall of the formation and a pump assembly in fluid communication with the probe assembly, both assemblies being within the apparatus. In a particular embodiment, the pump assembly is fluidly interconnected between the probe assembly and the sample cavity, whereby the pump assembly draws formation fluid via the probe assembly and delivers the formation fluid to the sample cavity.
In another embodiment, wherein the sample chamber includes a floating piston slidably positioned therein so as to define the sample cavity and a buffer/pressurization cavity, and the buffer/pressurization cavity is precharged with a buffer fluid, the pump assembly is fluidly interconnected between the buffer/pressurization cavity and a flow line within the apparatus. In this manner, buffer fluid is drawn from the buffer/pressurization cavity to create a pressure differential across the piston, thereby drawing formation fluid into the sample cavity.
Another method provided by the present invention induces formation fluid into the sample chamber by connecting the buffer cavity of the sample module, via the primary flowline, to another cavity or module which is kept at a pressure lower than the formation pressure, typically atmospheric pressure.